On a shortbox caller you want the lid to run flatter without canting much and the #10 keeps the lid steady . On a paddle call you are looking for the paddle to be loose and cant heavily to either side . This allows the lid to vibrate more . Therefore you use a smaller screw , i.e. a #8 along with a slightly loose hole. And you'll find that the screwhole in the lid of a shortbox is a tighter fit, in most cases , which also lends to it staying flat when you run it .